1. Field of the Invention
The disclosed invention is an improvement upon the methods of manufacture of capacitors of the type formed by plural windings of metallized thermoplastic films or sheets.
2. Prior Art
Metallized thermoplastic capacitors are formed from sheets of thermoplastic dielectric materials. The sheets are coated with a conductive material such as aluminum or zinc and wound into a coil. The conductive coatings are electrically paired, thereby forming the two plates of the capacitor. In winding the sheets into a coil it is the usual practice to offset the edge of the layers containing the conductive coatings which will be used for one element of the capacitor with respect to the remaining layers. In this way the layers which will be electrically connected together to form an element of the capacitor are axially offset at one end of the capacitor coil. At the other end of the coil the conductive layers which will form the other element of the capacitor will be similarly offset. By so offsetting the dielectric sheets in the coil, it is easier to electrically connect the lead electrodes to the conductive coatings.
After winding the capacitor into a coil it is traditionally clamped about its axis and heated for an extended period of time, causing the thermoplastic dielectric films to contract slightly creating a mechanically stable package. It has been found that when such heating is done the offset edges of the dielectric films tend to curl and close up the gaps between them.
After the heat treatment, a "schooping" process is performed, whereby a conductive material is bonded to the conductive coatings which are offset on each end. The usual practice in the industry is to flame solder a low melting temperature alloy onto the axial ends of the capacitor. Flame soldering consists in the spraying of molten metal onto the axial ends of the device. The gaps between the layers offset on each end are allowed to partially fill with the schooping material. The schooping material, being conductive, forms an electrical contact with the desired conductive coatings. In addition lead electrodes may be more readily soldered to the schooping material than to the fragile conductive coatings. Lead electrodes are then attached to the schooping material.
Because of the edge curl of the dielectric films due to the one step heat processing, the schooping material is usually unable to penetrate between the offset layers on the end of the capacitor as deeply as would be desired. This lack of penetration causes the capacitor to be restricted in the range of current densities for which it may be used. The lack of penetration causes an electrical connection having higher resistance than desirable to be formed between the schooping material and the conductive films, restricting the range of applications for capacitors manufactured by this method.